The present invention relates generally to communications, and more specifically to a novel and improved method and apparatus for step two W-CDMA searching.
Wireless communication systems are widely deployed to provide various types of communication such as voice, data, and so on. These systems may be based on code division multiple access (CDMA), time division multiple access (TDMA), or some other modulation techniques. A CDMA system provides certain advantages over other types of systems, including increased system capacity.
A CDMA system may be designed to support one or more CDMA standards such as (1) the xe2x80x9cTIA/EIA-95-B Mobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular Systemxe2x80x9d (the IS-95 standard), (2) the standard offered by a consortium named xe2x80x9c3rd Generation Partnership Projectxe2x80x9d (3GPP) and embodied in a set of documents including Document Nos. 3G TS 25.211, 3G TS 25.212, 3G TS 25.213, and 3G TS 25.214 (the W-CDMA standard), (3) the standard offered by a consortium named xe2x80x9c3rd Generation Partnership Project 2xe2x80x9d (3GPP2) and embodied in a set of documents including xe2x80x9cC.S0002-A Physical Layer Standard for cdma2000 Spread Spectrum Systems,xe2x80x9d the xe2x80x9cC.S0005-A Upper Layer (Layer 3) Signaling Standard for cdma2000 Spread Spectrum Systems,xe2x80x9d and the xe2x80x9cC.S0024 cdma2000 High Rate Packet Data Air Interface Specificationxe2x80x9d (the cdma2000 standard), and (4) some other standards.
Pseudorandom noise (PN) sequences are commonly used in CDMA systems for spreading transmitted data, including transmitted pilot signals. The time required to transmit a single value of the PN sequence is known as a chip, and the rate at which the chips vary is known as the chip rate. CDMA receivers commonly employ RAKE receivers. A rake receiver is typically made up of one or more searchers for locating direct and multipath pilots from one or more base stations, and two or more multipath demodulators (fingers) for receiving and combining information signals from those base stations.
Inherent in the design of direct sequence CDMA systems is the requirement that a receiver must align its PN sequences to those of a base station. Some systems, such as those defined by the W-CDMA standard, differentiate base stations using a unique PN code for each, known as a primary scrambling code. The W-CDMA standard defines two Gold code sequences for scrambling the downlink, one for the in-phase component (I) and another for the quadrature (Q). The I and Q PN sequences together are broadcast throughout the cell without data modulation. This broadcast is referred to as the common pilot channel (CPICH). The PN sequences generated are truncated to a length of 38,400 chips. The period of 38,400 chips is referred to as a radio frame. Each radio frame is divided into 15 equal sections referred to as slots. W-CDMA base stations operate asynchronously in relation to each other, so knowledge of the frame timing of one base station does not translate into knowledge of the frame timing of any other base station.
It is possible to search for W-CDMA base stations offset by offset (38,400 of them) for each of the 512 primary codes. However, this is not practical due to the excessive amount of time such a search would require. Instead, the W-CDMA standard calls for base stations to transmit two additional synchronization channels, the primary and secondary synchronization channels, to assist the subscriber unit in searching efficiently. As a result, W-CDMA search can be performed in three steps, which will be detailed more fully below.
For initial acquisition, the three-step W-CDMA search provides a great performance increase, in terms of reduced search time, over the impractical alternative of searching the entire PN space for each scrambling code. Search time is an important metric in determining the quality of a CDMA system. Decreased search time implies that searches can be done more frequently. As such, a subscriber unit can locate and access the best available cell more often, resulting in better signal transmission and reception, often at reduced transmission power levels by both the base station and the subscriber unit. This, in turn, increases the capacity of the CDMA system (either in terms of support for an increased number of users, or higher transmission rates, or both). Furthermore, decreased search time is also advantageous when a subscriber unit is in idle mode, a low-power state where a subscriber unit is not actively transmitting or receiving voice or data, but is periodically monitoring the system. Reduced search time allows the subscriber unit to spend more time in the low power state, thus reducing power consumption and increasing standby time.
W-CDMA searchers designed to reduce search time will accrue the benefits just described. In addition, however, efficiency of implementation is also important to reduce integrated circuit area and power consumption. Step two of the 3-step search method described above is a complex procedure. There is therefore a need in the art for efficient searchers that can perform step two W-CDMA searching, and similar procedures.
Embodiments disclosed herein address the need for efficient searching in asynchronous systems such as W-CDMA. In one aspect, a plurality of codes, such as SSCs, are correlated with a received signal at a plurality of offsets to produce a code/slot energy corresponding to each code/slot boundary pair. Unique subsets of the code/slot energies are summed to produce code sequence energies, the maximum of which indicates a located code sequence and slot boundary. In another aspect, the correlation is performed by sub-correlating the received signal with a common sequence, and performing a Fast Hadamard Transform (FHT) on the results. In yet another aspect, one sub-correlator can be used to search a plurality of peaks simultaneously. Various other aspects of the invention are also presented. These aspects have the benefit of circuit area and search-time efficiency which translate into reduced costs, increased standby time, increased acquisition speed, higher quality signal transmission, increased data throughput, decreased power, and improved overall system capacity.
The invention provides methods and system elements that implement various aspects, embodiments, and features of the invention, as described in further detail below.